The invention relates to a multistage countercurrent crystallization apparatus for multistage crystallization by adiabatic recrystallization and extractive washing, comprising a vertical column, the one end of said column being connected to a crystallizer and the other end thereof to a melter or a dissolver, said column furthermore containing a number of perforated plates, substantially spherical bodies contacting said perforated plates and cooperating with said perforated plates while being capable of moving with respect to said perforated plates, as well as a vibrator for putting the perforated plates into substantial vibration.
In the chemical industry, purification and concentration treatments play an important role. The purity of starting materials is subject to ever increasing purity requirements. In some instances purities of more than 99.99% are required. Among the known methods for purification, mention may be made of distillation, extraction and crystallization.
In principle, the application of crystallization offers a number of advantages for purification with respect to those of distillation, that is, the separating action of the singular process is often larger upon crystallization than upon distillation, the operating temperature is, in general, lower while the energy consumption is less.
Distillation and extraction are utilized on large scale because of the availability of devices and methods by means of which multi-stage separations may be carried out continuously. This is not the case for crystallization on industrial scale.
Organic compounds having a purity, for instance of 80-95%, which may be considered for a final purification will often contain a number (5 to 20 types) of contaminations. In many cases the separation factor for all of these contaminations is so high that only one crystallization suffices for obtaining crystals of the pure main component after a final washing treatment. However, it will then be required that the crystallization occurs at the equilibrium conditions. This is only possible if low crystallization velocities of, for example, 1-5 mm per unit of cooling surface area and per hour are utilized. Furthermore, adsorption of contaminations should not take place on the crystals and no contaminations forming mixed crystals may be present.
For an industrial application it is of importance that the crystallization velocities are high and amount to, for example, 100 mm per unit of cooling surface area per hour because in this way the volume of the installation and, consequently, also the installation costs related to the production output may be kept sufficiently small.
A further troubling factor is that the crystals should preferably be formed at the site having the highest concentration of contaminations (10 to 35% by weight). The higher the concentration of contaminations, the greater the risk of the formation of dendritic crystals and inclusions will be.
Even at crystallization velocities of 10 mm per unit of cooling surface area per hour, kinetic effects will play a dominating role, and such increasingly if the concentration of contaminations is higher. Consequently, no dense crystals of the pure main component will be formed, that could be washed and subsequently filtered or separated easily but only spongy structures having occlusions of many contaminations.
The aim is to obtain large dense crystals. Accordingly, it is necessary to utilize low growing velocities under very carefully selected conditions, thus leading consequently to sizable installations and high installation costs.
The Dutch Patent specification No. 158, 701, corresponding to commonly assigned U.S. patent application Ser. No. 194,945, filed Nov. 2, 1971, now abandoned in favor of its continuation-in-part Ser. No. 606,341, filed Aug. 21, 1975, now abandoned in favor of its continuation-in-part Ser. No. 768,145, filed Feb. 14, 1977, now U.S. Pat. No. 4,257,796, describes a vertically positioned countercurrent crystallization apparatus for multi-stage crystallization comprising a column containing a plurality of horizontal perforated plates positioned in a stacked position at regularly spaced intervals and having at the one end thereof a crystallization zone and at the other end thereof a melting or dissolution zone, said column being constructed mainly as a continuous adiabatic exchange column in which all separations between two successive compartments are of such a type that the passage of liquid as well as of crystals is possible and in which means have been provided for setting a large number of freely resting substantially spherical bodies into such a vehement vibration with respect to the perforated plates that they will regularly leave the perforated plates and return thereto while subjected to accelerating forces of about 2 to 5 times the gravitational force.
The U.S. Pat. No. 3,392,539 discloses a similar column in which by means of a succession of hot and cold zones an intermittent, i.e. not adiabatic, process of recrystallization and redissolution is made possible. Between the successive zones there have been provided only insulating plates passing liquid superimposed at some distance by a perforated plate on which there has been provided a crystal support consisting of several layers of balls. There is no desire to effect a passage of the crystals through these layers of balls. An overflow pipe has been provided for the transfer of these crystals from a cold crystallization zone to a hot dissolving zone arranged therebelow. The substantially saturated solution formed in a dissolving zone is transferred to a cold crystallization zone arranged therebelow via a by-pass provided with a pump. Means may be provided for imparting a pulsation to the mass of the liquid in the column which means may consist of a reciprocating piston in a side tube connected to the bottom of the column.
In accordance with the Swiss Patent specification No. 466,222, corresponding to U.S. Pat. No. 3,501,275, a reciprocating pulsation at the frequency of 5-500 min.sup.-1 and a stroke preferably of 200 mm (with a column length of 1 m ) is transmitted to the liquid in a column by application of a piston mechanism installed in a side tube. A drawback of this column is that the variable process parameters, such as crystal size, mixing, and density of packing cannot be adequately controlled. Moreover, this column does not provide enough possibilities of preventing crystal masses from adhering to the wall or of breaking up large crystal agglomerates.